2 月 java.util.Calendar 的一个奇怪行为

I faced with a strange behavior from java.util.Calendar:

import static org.junit.Assert.*;
import org.junit.Test;

import java.util.Calendar;

public class Tester1 {
    @Test
    public void test_monthOfDate() {
        assertEquals(1, monthOfDate(2013, 1, 30)); // OK
        assertEquals(1, monthOfDate(2013, 1, 31)); // OK

        // Start of February
        assertEquals(2, monthOfDate(2013, 2, 1));  // FAIL
        assertEquals(2, monthOfDate(2013, 2, 28)); // FAIL
        // to the end of it

        // and after that it is okay also
        assertEquals(3, monthOfDate(2013, 3, 1));  // OK
    }

    public int monthOfDate(int year, int month, int day) {
        Calendar cal = Calendar.getInstance();
        cal.set(Calendar.YEAR, year);
        cal.set(Calendar.MONTH, month - 1);

        // just a simple get! but seems it is very important
        cal.get(Calendar.MONTH);
        //

        cal.set(Calendar.DAY_OF_MONTH, day);

        return cal.get(Calendar.MONTH) + 1;
    }
}

我想知道为什么会这样?